Polyolefins are versatile materials used on a million-ton per year scale due to their excellent performance/price position. In some applications, particularly in physically demanding applications, the market needs are addressed by blending two or more polyolefin components. Examples include impact copolymers (ICPs) that comprise a polypropylene continuous phase and a rubber, typically ethylene-propylene copolymer, dispersed phase. These blends often combine high-melting stiff polymers and low-melting or amorphous soft polymers to tune the stiffness-toughness balance of the product. Current processes may blend these components in a compounding extruder. Compounding extruders are expensive machines and typically use significant amounts of energy. In other instances, the components are combined in the polymerization process. However, some of the blend components may be subpar due to the compromises forced by the current technology, such as in the case of ICPs in which it is virtually impossible to tune the rubber component to an optimal composition, composition distribution, and/or molecular weight.
Many current solution processes utilize adiabatic continuous stirred tank reactors (adiabatic CSTRs). These reactors have at least three disadvantages: (1) they require expensive chilling of the feed to absorb the reaction heat and thus allow acceptable single-pass conversion; (2) they are usually limited by agitation considerations to solution viscosity of a few thousand centipoise, which in turn limits single-pass conversion and polymer concentration in the reactor effluent thus increasing the solvent recycle load and cost per unit mass of product; and (3) operate at high pressures to keep all components in the liquid phase and thus the reactor liquid filled. Other conventional reactor designs use internal heat exchangers or external heat exchanger loops to remove the reaction heat. However, these reactors are also expensive to build and operate, and can have solution viscosity limitations and heat transfer surface fouling problems, especially when making high-melting polyolefins, particularly polypropylenes. The manufacturing of such products often requires relatively low temperatures, such as between 85° C. and 140° C. The lower temperature limit takes into account the tendency of the polymer to come out of solution as a solid thus fouling the reactor, the upper temperature takes into account catalyst stability and/or product properties, such as molecular weight and/or tacticity/stiffness/melting properties. Lower temperatures tend to yield higher molecular weight and higher-melting polyolefins; the latter particularly applies to polyolefins of high propylene contents, like polypropylene. These operational limits force the use of large and thus expensive heat exchangers due to the limitations in the temperature of the cooling medium since excessively cold surfaces tend to trigger fouling. The limited temperature difference thus forces the use of increased heat exchange surfaces for removing the reaction heat, which makes the equipment more expensive to fabricate and operate.
Boiling pool reactors have been disclosed in U.S. Pat. Nos. 2,885,389; 2,918,460; 3,126,460; and GB Patent 826,562. In the processes disclosed, elongated vertical reactors are operated in a slurry mode at a pressure and temperature such that the hydrocarbon continuous phase boils, and is subsequently condensed and returned to the reactor.
U.S. Pat. No. 6,716,936 discloses the use of two or more light solvent boiling pool reactors in series for the polymerization of ethylene and comonomer(s) to produce bimodal polyethylene copolymers having lower densities than polyethylenes made using conventional stirred tank, slurry loop or gas phase reactor technologies where the polymer is not dissolved in the reaction medium.
U.S. Pat. No. 7,423,100 discloses polymerization processes to produce polymers utilizing boiling pool reactor systems and diluents including hydrofluorocarbons where the polymer is present in the reactor as solid particles in a slurry.
U.S. Pat. No. 4,501,865 discloses a process to remove quantities of heat from exothermically running polymerization reactions of vinyl monomers in heterogeneous phase, by adding liquids to the reaction medium for the removal of heat and to regulate the reaction temperature, which liquids do not dissolve the polymer under the reaction conditions and the boiling temperatures of which are lower than or are identical to the technically predetermined reaction temperatures under the polymerization conditions which are applied, and the heat which is released in the reaction system is removed by evaporating these liquids.
US 2014/0213745 discloses a method to prepare, and compositions pertaining to, an amorphous polymer comprising: at least 95 mol % propylene and 0 to 5 mol % vinyl monomer content, wherein the polymer has a g′vis of less than 0.95, an Mn of about 200 to about 10,000, an ΔHf of less than 10 J/g and has greater than 50% allylic chain end functionality. At page 10, paragraph 137, US 2014/0213745 states that “[a]ny bulk, homogeneous solution, boiling pool or slurry process known in the art can be used.”
U.S. Pat. No. 8,058,371 discloses processes for polymerizing propylene, where: 1) about 40 wt % to about 80 wt % propylene monomer, based on total weight of propylene monomer and diluent, and about 20 wt % to about 60 wt % diluent, based on total weight of propylene monomer and diluent, can be fed into a reactor; 2) the propylene monomer can be polymerized in the presence of a metallocene catalyst and an activator within the reactor at a temperature of about 80° C. or more and a pressure of about 13 MPa or more to produce a polymer product in a homogenous system; and 3) about 20 wt % to about 76 wt % (preferably about 28 wt % to about 76 wt %) propylene monomer, based on total weight of the propylene monomer, diluent, and polymer product, can be present at the reactor exit at steady state conditions.
U.S. Pat. No. 7,807,769 discloses a supercritical process to make isotactic propylene homopolymer having: 1) more than 15 and less than 100 regio defects (sum of 2,1-erythro and 2,1-threo insertions and 3,1-isomerizations) per 10,000 propylene units; 2) an Mw of 35000 g/mol or more; 3) a peak melting temperature of greater than 149° C.; 4) an mmmm pentad fraction of 0.85 or more; 5) a heat of fusion of 80 J/g or more; and 6) a peak melting temperature minus peak crystallization temperature (Tmp−Tcp) of less than or equal to (0.907 times Tmp) minus 99.64 (Tmp−Tcp<(0.907×Tmp)−99.64), as measured in ° C. on the homopolymer having 0 wt % nucleating agent.
U.S. Pat. No. 7,812,104 discloses a process for producing a propylene-based olefin homopolymer or copolymer, where a monomer composition comprising propylene is contacted with a polymerization catalyst system under homogeneous polymerization conditions (such as solution, supersolution or supercritical conditions), wherein the polymerization catalyst system includes an activator and a bridged bis-indenyl transition metal (group 4) compound substituted with a carbazole (unsubstituted or substituted) at the 4 position.
Additional references of interest include: WO 1993/021241 and US 2013/0203946. None of the above disclose processes that can make both high-melting polymers, such as, for example, polypropylenes and soft, low melting polymers, such as, for example, ethylene-propylene or ethylene-(C4-C12 alpha olefin) rubbers in an efficient, non-fouling, low-cost process operating with homogeneous polymerization phases and utilizing coordination catalysts (such as single-site catalysts). Thus, there is a need for a clean, efficient, rapid, and low-cost process to make polyolefins and polyolefin blends comprising high-melting and/or low-melting components with enhanced properties in a continuous solution process where the heat of polymerization is removed without causing fouling of the reactor.
The current invention overcomes the limitations discussed above, thus allowing reduced investment and operating costs while also increasing operation reliability by operating the reactor in such a way (e.g., at or near the boiling point of the reaction medium) as to remove the reaction heat by evaporative means. The heat removal rate can be readily controlled by the rate of reflux, a method typically employed in refinery and petrochemical separations. These isothermal reactors thus operate with a homogeneous reaction medium enabling the optimal deployment of coordination catalysts (such as single-site catalysts). They also operate at lower pressures than the liquid filled conventional reactors making them cheaper to build and operate, and can often operate at higher solution viscosity than other designs owing to the agitation provided by the boiling liquid medium. Thus, the current invention provides a process that can produce polymers at higher concentrations while typically having less fouling than current processes.
The process of the instant invention also overcomes the deficiencies discussed above for ICP's by producing the stiff and soft components in separate reactors (one or both of which may use evaporative cooling) in which the process conditions and catalysts are optimized for the two different (stiff and soft) components. Further improvement, particularly for the soft component is also achieved by production in a homogeneous solution process utilizing coordination catalysts (such as single-site catalysts) homogeneous catalyst and, preferably evaporative cooling. Such operation mode and catalysts afford precise composition and molecular weight control and narrow molecular weight and composition distribution of the soft component, all useful to achieving the maximum benefit this component provides for the final product blend. A further benefit of the production of the stiff and soft components in the currently-disclosed separate, independently tunable reactors is the ability to precisely control the molecular weight and melt viscosity of the blend components. This in turn allows the development of the desired high and uniform dispersion of the soft product component in the continuous stiff phase affording the best utilization of the soft component for improving the toughness with the minimal degradation of the stiffness of the final product blend.
Further, the processes disclosed herein have: (1) improved/lower cost cooling due to heat exchange in the absence of polymer, which avoids fouling and increases heat exchange efficiency due to lower viscosity, and (2) higher polymer concentration in the reactor without reducing mixing, and thus without jeopardizing product quality, due to the mixing enhancement from the churn caused by boiling.
The processes of the instant invention operate with coordination catalysts (such as single-site catalysts) in a homogeneous reaction medium, free of slurry, and free of heterogeneous catalyst.